1. Field of the Invention
This invention relates to a laser diode and more particularly to a stripe-type laser diode having an improved efficiency of the optical confinement.
2. Description of the Prior Art
A conventional stripe-type laser diode is described by K. Aiki et al. in Applied Physics Letters, Vol. 30, Nov. 12, 1977, pp. 649-651.
Preferring to FIG. 1, another conventional stripe-type laser diode is shown in a sectional view normal to the direction of light propagation. Formed over a main surface of a p-GaAs substrate 1 is a current constricting n-GaAs layer 2 with a slit 3 for constricting electric current within the slit 3. The slit 3 is opened through the constricting layer 2 by etching and separates the constricting layer 2 into two areas. A lower clad layer 4 of p-Ga.sub.0.6 Al.sub.0.4 As is epitaxitially formed over the constricting layer 2, burying the slit 3. An active layer 5 of p-Ga.sub.0.9 Al.sub.0.1 As, an upper clad layer 6 of n-Ga.sub.0.6 Al.sub.0.4 As and a surface layer 7 of n-GaAs are epitaxially formed in order over the lower clad layer 4. An N-electrode 8 formed on the surface layer 7 is supplied with a negative potential to cause the diode to emit a laser beam. On the other hand, a P-electrode 9 on the opposite surface of the substrate 1 should be supplied with a positive potential. An emission area 10 in the active layer 5 just above the slit 3 emits light due to the combination of electrons and positive holes when the forward voltage is applied between the P-electrode 9 and the N-electrode 8.
FIG. 2 schematically shows an energy band profile as to electrons and holes in the lower clad layer 4, active layer 5 and upper clad layer 6 with a forward voltage between the P-electrode 9 and the N-electrode 8. Due to the compositional difference between the active layer of Ga.sub.0.9 Al.sub.0.1 As and the clad layers of Ga.sub.0.6 Al.sub.0.4 As, the band gap in the active layer is smaller than that in the clad layers, as seen in FIG. 2. Therefore, electrons and holes are accumulated in the active layer, and then they readily combine with each other across the smaller band gap. Consequently, photons due to the combination of electrons and holes are emitted dominantly in the active layer.
Those photons are emitted in the emission area 10 of the active layer 5 just above the slit 3, since the injection current can flow only through the slit 3 and is obstructed in the other area by the constricting n-layer 2 interposed between the p-substrate 1 and the lower clad p-layer 4. Thus, the emission area 10 in the active layer 5 has a stripe geometry along the slit 3. Light waves thus locally generated should be further amplified, and the amplifying efficiency largely depends on efficiency of the optical confinement within the emission region 10 of the stripe geometry. The optical confinement may be achieved by a refractive index of the stripe region which is higher than that of the surrounding region.
In the instance shown in FIG. 1, light waves generated in the emission region 10 is confined within the active layer 5, since the refractive index in the active layer 5 of Ga.sub.0.9 Al.sub.0.1 As is higher than that in the lower and upper cald layers 4, 6 of Ga.sub.0.6 Al.sub.0.4 As due to the compositional difference. However, the light waves should be confined not only within the active layer 5 but also within the width of the stripe region 10.
When the lower clad layer 4 and the active layer 5 are thin enough, the effective refractive index of the active layer 5 can be controlled and lowered by the constricting layer 2 which is optically absorptive. In the active layer 5, therefore, the refractive index of the stripe region 10 just above the slit 3 is effectively higher than that of the other region accompanied by the absorptive layer 2. Consequently, the light waves are confined within and guided along the stripe region 10. Such control of the optical confinement within the width of the stripe region 10 will be discussed in more details below.
FIG. 3A is an enlarged fragmentary view in the vicinity of the slit 3 of the diode shown in FIG. 1, and FIG. 3C schematically shows distribution profiles of free electric charge in the active layer 5 in the vicinity of the slit 3 correspondingly to FIG. 3A. When the injection current is small, the density of free electric charge is somewhat raised in the stripe region 10 just above the slit 3 as shown by a chained line A in FIG. 3C. Consequently, a component of the refractive index which is inversely proportional to the density of electric charge is somewhat lowered in the stripe region 10 as shown by a chained line A in FIG. 3D. On the other hand, another component of the refractive index which is lowered by the optical absorptive effect of the constricting layer 2 is shown by a solid line A in FIG. 3E. The refractive index resulted from those two components is shown by a chained line A in FIG. 3F. As will be understood from the chained line A in FIG. 3F, the refractive index in the stripe region 10 (right hand side of a point P.sub.2) is higher than that in the other region (left hand side of a point P.sub.1), so that the optical waves are confined within the width of the stripe 10.
However, when the injection current is large, the density of free electric charge is considerably raised in the stripe region 10 as shown by a broken line B in FIG. 3C. Consequently, the component of the refractive index which is inversely proportional to the density of electric charge is considerably lowered in the stripe region 10 as shown by a broken line B in FIG. 3D. On the other hand, the influence of the optically absorptive layer 2 on the other component of the refractive index is left unchanged as shown by a solid line B in FIG. 3E which is the same as the line A. The refractive index resulting from those two components is shown by a broken line B in FIG. 3F. As will be seen from the broken line B in FIG. 3F, the refractive index in the stripe region 10 (right side of a point Q.sub.1, including a point Q.sub.2) is not much higher than that in the other region (left side of a point Q.sub.0), so that it is difficult to completely confine the light waves within the width of the stripe 10.